1. Field of the Invention
The present invention relates to a steering wheel in which a rim cored bar portion, a hub cored bar portion, and a spoke cored bar portion are covered with urethane.
2. Description of the Related Art
In so far as a steering wheel of a motor vehicle is concerned, its related cored bar portions having an aluminum alloy, a magnesium alloy or the like casted therein are covered with a synthetic resin to thereby form external shapes (which are also referred to as volumes) of a rim portion and a spoke portion.
That is, the steering wheel has: a rim in which a rim cored bar portion formed in a shape of a circular ring is covered with urethane serving as a synthetic resin, and which is gripped by a driver; a hub in which a hub cored bar portion disposed at a center part in a radial direction of the rim is covered with urethane; and a spoke in which at least one spoke cored bar portion configured to couple the hub cored bar portion and the rim cored bar portion to each other in a radial direction is covered with urethane. At the time of covering the cored bar portions with urethane, there is a need to provide a core material inside of a portion with a predetermined degree of volume. This provision is indispensable to ensure strength of molded articles or to take an appropriate countermeasure against shrinkage or the like. It is to be noted that, in so far as a resin configured to cover the cored bar portion is concerned, in a case where the resin is of such a type that a hard resin such as polypropylene is employed, there is a problem that its related surface hardness is great and its related touch sense is poor when the rim portion is gripped.
On the other hand, in so far as a resin of such a type that the cored bar portion is covered with urethane is concerned, an appropriate softness in touch sense can be imparted.
However, urethane itself is a resin having a low strength thereof; and therefore, inside of the resin, a cored bar portion made of a metal or the like having a high rigidity thereof needs to be disposed over its related detailed parts. It is to be noted that, if urethane is molded at a portion at which the cored bar portion is not provided, the strength of urethane with respect to an external force cannot be guaranteed.
Here, in a conventional steering wheel, a spoke is of such a type that a comparatively small amount of urethane is employed.
Now, with reference to FIG. 12 (a), there is shown a case in which a rib for preventing shrinkage of a resin is not provided at an inclined coupling portion 13aa which is a part of a spoke cored bar portion 12a of a steering cored bar portion 10a. Also, with referring to the figure, there is illustrated a case in which, in so far as urethane serving as a resin configured to cover the steering cored bar 10a is concerned, even if there is not provided a rib which is small in amount of urethane configured to cover the spoke cored bar portion 12a, and which is for preventing the shrinkage of urethane, the appearance quality of urethane meets a predetermined criterion (reference should be made to Japanese Unexamined Patent Application Publication No. 2013-95224, for example). It is to be noted that reference numeral 11a in the figure designates a rim cored bar portion.
The steering wheel that is described in Japanese Unexamined Patent Application Publication No. 2013-95224 is characterized as the one having a large thickness portion which is thick at a center portion configured to engage with a steering shaft of a hub cored bar portion and a small thickness portion which is gradually smaller in thickness as the steering wheel goes to an end part thereof.
In another conventional steering wheel, a spoke is of such a type that a comparatively large amount of urethane is employed.
Now, with reference to FIG. 13, there is shown a case in which a rib 24 for preventing the shrinkage of urethane is provided at an inclined coupling portion 23aa of a steering cored bar 20a. Also, with reference to the figure, there is shown a case in which in urethane configured to cover the steering cored bar 20a, a large amount of urethane is employed as a covering material of a spoke 22, and there is a need to provide a rib 24 for preventing the shrinkage of urethane. In addition, there is shown a case in which such a rib 24 is formed in the inclined coupling portion 23aa of the steering cored bar 20a, whereby the appearance quality and the dynamic performance of urethane meet a predetermined criterion.
Hereinafter, the dynamic performance of a steering wheel will be described.
The performance of the steering wheel is characterized by having a function of enabling a rib and a spoke cored bar portion to be deformed to an extent such that the rim and the spoke cored bar portion are not separated from each other in a case where a predetermined external pressure is applied to the steering wheel.
In an impact test, it is required that the spoke cored bar portion bends by some tens of millimeters without a breakage of a rim cored bar portion or a hub cored bar portion of a steering coed bar, or alternatively, that a rib is not separated from an inclined coupling portion of the spoke cored bar portion.
In so far as the steering cored bar 20a shown in FIG. 13 is concerned, a load is applied in a direction which is inclined with respect to a steering rotary shaft, whereby a rim portion is deformed significantly to a lower side with respect to a hub cored bar portion 23a in a region which is indicated by the dashed line L5 to thereby able to suppress generation of an excessive resistance force.
In the meanwhile, in recent years, there has been a tendency that a variety of switches or members are provided in a steering wheel, or alternatively, a thickness of urethane is increased in order to produce a high grade sense.
If the thickness of urethane is increased, the amount of shrinkage at the time of execution of a urethane covering process is increased, accordingly. Thus, an increased size of a rib is taken into an account.
However, if the rib is increased in size, although the shrinkage associated with an increased quantity of urethane is prevented, there is an apprehension that a predetermined criterion cannot be met in an inclined load characteristic or in a flat push test.
In addition, if the molding requirement mentioned above (in other words, a requirement that a cored bar portion is formed inside of a resin) is merely applied to the steering wheel, the following problem occurs.
A spoke portion is inclined with respect to a vertical direction (an axial direction) of a rim portion and a boss portion and thus an inclined coupling portion which is formed at a cored bar portion is obtained as a portion of which a rigidity is increased with respect to a load acting on the steering wheel, and its related resistance force becomes excessive.